1. Field of the Invention
The present invention relates to GaAs wafers and particularly to a method for realizing nondestructive, whole-wafer dislocation maps of a polished conducting GaAs wafer.
2. Description of the Prior Art
Bulk n+GaAs:Si wafers are used to fabricate a variety of devices such as lasers and solar cells. Up to this time, very little attention has been given to GaAs: Si whole-wafer materials characterization, in contrast to the case of semi-insulating (SI) GaAs, where a variety of whole-wafer characterization techniques have been developed. For the SI case, two of the important whole-wafer parameters are EL2 and dislocation (or etch pit) density.
Transmission maps have proved to be very useful in delineating variations of EL2 concentration, dislocation density, and stress in semi-insulating (SI) GaAs wafers. It has been possible in some cases to correlate transmission maps of material properties with maps of device properties from the same or an adjacent wafer. While the EL2 and stress measurements are nondestructive, the dislocation density measurements require a KOH etch to form etch pits at the surface. In contrast to the EL2 centers, which reduce the transmission by absorption, the etch pits reduce the transmission by scattering, however, both effects are easily quantifiable.
U.S. Pat. No. 5,008,542 discloses a method and system for measuring whole-wafer etch pit or dislocation density (.rho.D) in which an etched GaAs wafer is tested for fractional transmission at a plurality of points over its surface. The fractional transmission (T) of light through the wafer is detected at a plurality of points, digitized and fed to a computer for storage. Two or more of the transmission values are selected for calibration and are compared with manually counted etch pit densities at the same locations on the wafer. From this calibration, together with an estimate of the average etch pit size (area), the values for fractional transmission in all regions of the wafer are converted directly to etch pit density.
A long sought goal in the semiconductor industry has been a convenient technique for nondestructive evaluation of dislocation density. However, none of the usual optical techniques have proved to be useful for this purpose so far. For n+GaAs wafers used in the fabrication of lasers, solar cells, and other devices, it seems that very little whole-wafer characterization has been carried out. In n+GaAs, EL2 is very small and not important, but the dislocations are even more important than in SI GaAs because they can lead to dark-line defects and other nonradiative recombination centers which are deleterious to lasers.